Re: [Ifeffit] Ifeffit Digest, Vol 158, Issue 3

Yup. In fact, self absorption, in the correct sense, can help reduce "self-absorption" in the misnomer sense! That's how the grazing-exit method works. I might also note that although many elaborate schemes have been proposed for correcting for the effect, they all require knowing things about the sample that you don't always know, e.g. the exact geometry of something that isn't planar, or the exact composition. I find that the simple parameterization which comes from a thick-planar-sample model works reasonably well as an empirical method with an unknown parameter. Still, I wouldn't necessarily trust Debye-Waller (ss2) values taken from EXAFS on strongly overabsorbed samples, even with a correction that gets the right coordination number. mam On 4/3/2016 7:04 PM, Matt Newville wrote:

Hi Matthew,

On Sun, Apr 3, 2016 at 8:13 PM, Matthew Marcus mailto:mamarcus@lbl.gov> wrote:

<Rant> It shouldn't be called 'self-absorption'. That's a misnomer, which seems to have come from a 1992 paper (Troger, et. al."Full correction of the self-absorption in soft-fluorescence extended x-ray-absorption fine structure", PRB 46,3283 (1992). The effect was described and analyzed in a 1982 paper, which called it an "attenuation factor": Goulon, et. al. "On experimental attenuation factors of the amplitude of the EXAFS oscillations in absorption, reflectivity and luminescence measurements", J. Physique 43, 539 (1982). </Rant> mam

Thanks!! I completely agree, though I wasn't aware of the historical precedence for the mistake. Using "over-absorption" is a far better term.

In X-ray fluorescence, "self-absorption" actually means the attenuation of fluorescence generated within a sample as it travels out of the sample. For over-absorption in XAFS, the issue is measuring absorption in fluorescence mode when the concentration of the absorbing element is not infinitesimal or when the sample thickness is not infinitesimal.

--Matt

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